1. Field of the Invention
This invention concerns pneumatic oscillators, especially but not exclusively for the operation of resuscitators and/or ventilators and like devices for inducing or assisting lung function in human patients.
2. Background Discussion
In general, such devices generate a train of pulses of breathable gas that is ducted to a patient, usually via a so-called patient valve and/or an oronasal mask or tracheal intubation device. The generation of the pulse train with pulses at the required intervals and with appropriate tidal flow characteristics requires some form of switching mechanism controlling the flow of breathable gas from a source to the pulse output. Simplicity, reliability and constancy of performance in service, and robustness are dominant criteria in the design of resuscitators or ventilators, especially those intended for use by emergency services such as ambulance crews, or for use in a domestic environment by non-specialist operators.
3. The Prior Art
Pneumatic oscillators exist in various forms and many forms have been applied to this purpose. GB-A-1 533 550 exemplifies one such form of pneumatic oscillator that has been successfully applied in practice but even that oscillator comprises several components and several moving parts, with consequent complexity and cost.
In another form of pneumatic oscillator, a piston or its equivalent is reciprocable to open and close a flow path between a source of pressurized breathable gas and a pulse output. The piston is biassed towards the flow path-closing position and the biassing is supplemented by gas pressure derived from the output of the device. Source gas pressure is applied to the piston in a manner to overcome the biassing so as to cause the piston to move to its flow path-opening position; whereupon the device outputs a gas pressure pulse from which pressure is derived to supplement the biassing and restore the piston to its flow path-closing position. Examples of such an oscillator are disclosed in FR-A-1 530 478 and US-A-3 881 480.
To obtain a snap-action in the opening and closing of the flow path, a poppet valve arrangement is utilised in which a sealing lip coacts, in the flow path-closing condition, with a resilient facing in such a manner as to isolate an area of the piston or equivalent from the gas pressure acting elsewhere. Accordingly, when the flow path is open, gas pressure is applied to a different area than when the flow path is closed with the consequence that there is an abrupt change of effective area exposed to gas pressure at the point of switching, and an abrupt change in the force balance on the piston.
Although an oscillator of this general form exhibits remarkable simplicity and would appear to be eminently suitable for the applications considered, in practice such oscillators have not achieved widespread adoption because the attainment of accurate and reproducible performance characteristic depends critically upon the maintenance of close tolerances in manufacture and even then the characteristics tend to change, unpredictably, in service. The reason for this is that the characteristics are critically affected by the force balance on the piston or its equivalent at the point of switching and this in turn is influenced by a number of factors. One such factor is the relative areas of piston or equivalent exposed to gas pressure at the point of switching, as determined by engagement of the sealing lip of the poppet valve with its resilient facing.
For a good snap-action it is essential that there is minimal leakage across this seal right up to the point of force balance and that at this point, the flow path is opened suddenly and with a force/flow gap characteristic that is stable over long periods and is unaffected by wide ranges of temperatures and duty patterns. There is as a consequence a conflict of design requirements because to obtain good sealing (low leakage) the combination of a resilient facing and a sharply defined lip is desirable. However this combination leads to permanent set indentation and/or cutting of the facing material in service, with consequent deterioration of performance. Accordingly it is usual to depart from the ideal configuration for low leakage by employing a more rounded sealing lip. While this provides good sealing over long periods of service and avoids cutting of the facing material, it brings with it the disadvantage that the line of contact between the lip and the facing material is not sharply defined and varies in service as a result of the facing material becoming permanently indented. Thus the area within the lip tends to change during service and so alter the ratio of the areas that control the force balance at the point of switching.